Pedialyte is usually used when a kitten is sick with diarrhea or something. It's used to replenish lost electrolytes.
If your kitten isn't sick there isn't any reason why you would need to use Pedialyte. If your vet recommends it, of course, but I don't know of any reason to use it without indication of a problem.
I don't think it will hurt for the short term but I wouldn't recommend using it for a long time.
Is the kitten still with its mother and litter mates? That's probably the most important thing for a kitten at such a young age. They really need to be with their mother, practically at all times.
KMR is good to feed kittens as a supplement but mother's milk is best. That's where kittens get their first boost of antibodies to help their immune systems fight off infection.
It's also a good idea to offer some KMR to the mother, too. She just had a litter of kittens. It took a lot out of her. Her energy reserves are likely depleted. The extra calories from the KMR will help the mother recharge, so to speak.
Whetner it be mother or kittens, adding Pedialyte to the KMR might be okay for a boost but I don't think you need to use it all the time unless a vet says so, first.